The energy cost of walking is 30-50% higher in unilateral transfemoral amputees than in able-bodied controls, and at least half of this is due to the loss of knee function. Modern commercial knees with microprocessor-controlled damping mechanisms, such as the Rheo Knee (available from Össur Americas of Foothill Ranch, Calif.) and C-Leg (available from Otto Bock US of Minneapolis, Minn.), have only reduced the energy cost by 3-5%, compared to conventional passive prosthetic mechanisms. This suggests that an essential aspect of knee function is still missing. In studies of transfemoral amputee gait mechanics, it is noticeable that all prosthetic technologies (including microprocessor-controlled dampers) cause the patient to walk without knee flexion during the stance phase, whereas able-bodied subjects have about 15 degrees of flexion-extension movement. Stance phase knee flexion is one of the “six determinants of gait”, and although its importance is still debated, its consistent presence in able-bodied gait suggests that it is useful. Indeed, when able-bodied subjects are prevented from flexing their knee during the stance phase, they use 25% more energy for walking. Lack of stance phase knee flexion may also be responsible for gait asymmetry and compensatory strategies such as increased hip muscle forces, possibly leading to overuse injuries and osteoarthritis.
Even though controlled damper devices are designed to allow stance phase knee flexion, patients seem to avoid using this feature. This may be partly due to a lack of confidence in the stability of the limb against buckling. Another explanation may be the considerable relative movement between socket and residual limb, which makes the limb perhaps too compliant, even with a stiff knee. A third possible explanation is that a damper device will dissipate a considerable amount of energy when allowing a controlled flexion during the stance phase, and is not able to produce the required positive work for the subsequent knee extension. The hip extensors would be entirely responsible for bringing the knee back to extension during mid to late stance and for restoring the lost energy. While this strategy for achieving a kinematically normal gait is theoretically possible, it would be kinetically abnormal and require extraordinary effort, so it is understandable that patients seem to avoid this.
The lack of positive work for knee extension often poses a greater challenge for amputees participating in other activities besides level walking. During able-bodied running, there is about 40 degrees of stance phase flexion-extension, which is probably not feasible for users of current prosthetic devices based on what has been observed during walking. This requires transfemoral amputees to run with extreme asymmetries and they accordingly cannot approach able-bodied running speeds. Sit-to-stand is an important function, and transfemoral amputees perform this movement with near-normal kinematics but without any joint moment in the prosthetic knee, i.e., entirely powered by the sound leg. This is inevitable because controlled dampers cannot produce a knee extensor moment while the knee is extending. The most severe functional deficits are found during activities that require net positive work, such as walking uphill and stair ascent. Stair ascent requires large amounts of positive work at the knee which cannot be delivered with controlled damper devices. Consequently, transfemoral amputees are typically seen performing stair ascent with a step-by-step technique where the sound limb leads and the prosthetic limb follows passively.
In order to overcome the limitations of controlled damping devices, alternatives have been developed, but with limited commercial success to date. Most notably, the Power Knee (available from Össur Americas of Foothill Ranch, Calif.) actuates the knee with a direct drive motor. A similar concept, with more sophisticated control, has been described recently. Direct drive devices consume far more electrical power than controlled dampers, which limits their applicability. It has been shown that series elastic actuators can dramatically reduce the power requirements. These actuators allow some of the knee function to be delivered by passive springs, and the control timing can be such that the motor mainly moves when unloaded.
In most cyclic activities, such as walking, running, and a stand-sit-stand sequence, no net positive work is required at the knee, which suggests that a motor may not be needed. There are, however, alternating phases of negative and positive work. Therefore, energy must be stored during periods of negative work, rather than dissipated with a damper, and the stored energy must be released later when positive work is needed. A stiff knee extensor spring, such as in the XT9 (available from SymBiotechs USA of Saratoga Springs, Utah) provides functional energy storage and release in stance-only activities, but is not suitable for walking where the spring must be disengaged during the swing phase. Although large reductions in metabolic cost were reported in test subjects, a disengageable-spring device never appears to have been commercialized. It may be that the passive mechanism to control the stance-swing transitions was not sufficiently safe against buckling, or too specialized to allow activities other than walking.